In numerous applications, such as many in the automotive industry, a polymeric material that exhibits a good level of scratch resistance balanced with rigidity and impact toughness is desired. These properties tend to vary, however, such that efforts to enhance one will often result in deterioration of one, or both, of the other beneficial properties.
Polypropylene blends are useful in a wide variety of applications due to their strength, environmental resistance, and processability. While highly crystalline polypropylene does exhibit good mar and scratch resistance, it does not possess the impact toughness required in many important applications such as the making of automobile parts. Special polymeric materials have been developed that overcome this problem to some degree.
Attempts to remedy polypropylene's deficiency in impact toughness by blending with impact modifying copolymers of ethylene and other alpha-olefins, terpolymers of ethylene, other alpha-olefins, and dienes have not been completely successful. Elastomer modified polypropylene blends, also known as thermoplastic polyolefins, have the advantage of improved toughness, especially for cold temperature impact. They are widely used for formed or shaped articles, such as automotive parts, toys, furniture, and housing products. Although the impact toughness of those compositions is improved by these modifiers, the scratch resistance has been found to decrease. That is, the scratch resistance of polypropylene blends containing impact modifiers, such as ethylene-propylene copolymers, ethylene-propylene terpolymers, ethylene-butene copolymers, or ethylene-octene copolymers, is poor.
Increasing the crystallinity of the polypropylene to obtain a harder surface, and/or adding hard mineral filler to these blends, has been attempted as a countermeasure but without complete success. One conventional method to enhance surface characteristics is to use inorganic particulate material. Uniform dispersion of these particulates is difficult to achieve, however, and this tends to result in non-uniform surface properties in such products. The use of these particulates also tends to damage other desirable physical properties of the polyolefin, resulting in loss of impact strength and/or toughness. Debonding of such particulates from the polyolefin system also contributes to stress whitening, which is not desirable.
Another conventional way to enhance surface characteristics of various articles is to apply acrylic polymers or coatings to an article and subsequently cure the polymer or coating with a radiation source, such as ultraviolet radiation.
A method to enhance surface characteristics of polyolefins is described in U.S. Pat. No. 4,000,216, which discloses an extrudable, moldable, or heat formable blend of a thermoplastic polymer and a surface altering agent of at least one monoethylenically unsaturated monomer for said thermoplastic polymer, wherein the surface altering agent has cross-linked polymer particles having an average size of 1 to 30 microns. The surface altering agent is preferably prepared by an endopolymerization, which is used with a compatible polyolefin to be altered.
U.S. Pat. No. 5,880,198 describes thermoplastic resin compositions comprising polypropylene, a styrene containing elastomer, and talc, and having an acceptable balance of toughness and rigidity. The disclosure stresses the importance of the proportions of each component used to achieve this balance. In U.S. Pat. No. 6,384,122, a similar balance of properties is reported for a thermoplastic resin comprising an ethylene-propylene based polymer composition, an ethylene alpha-olefin copolymer based rubber and/or a rubber containing vinyl aromatic compounds and talc. Another example of a composition that provides an acceptable balance of toughness and rigidity is reported in Japanese Patent Application 10219040A for a resin composition consisting of a polyolefin based resin and a block copolymer based on aromatic vinyl and butadiene monomer units. Polymer blends which can be formed or shaped into lightweight and durable articles useful, for example, as automobile parts, toys, housings for various types of equipment, and the like, are well known in the art.
The physical and/or chemical properties of the thermoplastic polyolefin blends can be modified either by blending them with other thermoplastic polymers, or by incorporating into them materials having one or more polar groups, or both. For example, U.S. Pat. No. 4,946,896 describes a thermoplastic polyolefin comprising 20-80 weight percent polypropylene; 5-38 weight percent of an ethylene copolymer consisting of ethylene, an ester unit of either alkyl acrylate or methacrylate, and an unsaturated dicarboxylic acid anhydride; and 5-70 weight percent ethylene-propylene rubber. Similarly, U.S. Pat. No. 4,888,391 describes a polyolefin composition comprising a blend of a polyolefin as the continuous phase with an ethylene/acrylate/acrylic acid terpolymer as a discontinuous phase. These polyolefin-based blends are paintable. Another example of improving both scratch and impact resistance is reported in U.S. Pat. No. 6,423,779, where polypropylene and a polyphenylene oxide resin are blended together with a compatibilizer.
Despite these prior art formulations, there remains a need to obtain polymeric materials that have a good level of mar/scratch resistance along with the physical property requirements of rigidity, strength, processability, and low temperature impact toughness. The present invention provides certain blends that meet these needs.